The present invention relates to a wing tip extension for a wing, in particular for an aircraft wing.
Winglets or wing tip extensions for wings, in particular for aircraft wings, are known from the state of the art and serve to reduce and to minimize the induced drag arising at the wings when flying, which drag is brought about by the wake vortices at the wing ends. The drag of an aircraft wing of a passenger or transport aircraft in cruise flight is determined to a certain proportion through the so-called induced drag of the aircraft wing. This induced drag is brought about through a change of flow at the wing ends. The flow is deflected inwardly on the wing upper side and outwardly on the wing lower side. The cause of this is the pressure difference between upper and lower sides, which can equalize only at the wing tips. From this there arises a wake vortex which consumes considerable energy, that is causes drag. This wake vortex depends largely upon the geometry of the wing. The induced drag is thereby inversely proportional to the so-called aspect ratio, the ratio of the square of the span to the area of the wing. An increase of the aspect ratio therefore leads to a reduction of the induced drag. The increase of the aspect ratio has, however, other disadvantageous effects, such as for example an increase of the structural weight and an increase of the bending moments.
The configuration of a wing tip extension for a wing thus requires the optimization of a plurality of different and partly conflicting conditions. An important parameter is for example the sweep or sweepback angle of the wing and of the wing tip extension. Greater sweepback angles mean, as a rule, more favorable transonic characteristics during flight. A further factor to be taken into account is the frictional drag, which increases with increase of the area of the wing flowed over. Further, slow flight characteristics are significantly influenced by the geometry of the wing and also by wing tip extensions. Since, with increased cruising speeds, the difference between slow flight upon taking off and landing and cruise flight has become greater, means must be found in order to ensure a sufficient lift also in the case of slow flight. For this purpose, for example, leading edge slots and trailing edge flaps on the wing, and leading edge devices on the wing tip extension have been proposed; see for example in U.S. Pat. No. 5,039,032. The provision of additional slots or flaps, however, increases the complexity of the wing or of the wing tip extension considerably. Further, the geometry of the wing tip extension is to be so configured that elastic deformations do not lead to a disadvantageous flow-structure interaction.
It has been sought to provide solutions for various of the above-mentioned requirements by means of wing tip extensions proposed in the state of the art. For example, U.S. Pat. No. 6,089,502 proposes a wing tip extension for supersonic aircraft. The wing tip extension is in substance planar, that is, it extends in the plane of the wing and turns from the wing end strongly to the rear. In the connection region of the wing tip extension to the wings, leading and/or trailing edges have a pronounced sharp bend or kink. U.S. Pat. No. 5,039,032, already mentioned above, likewise proposes a planar wing tip extension for an aircraft wing, with which the sweepback angle of the wing tip extension changes abruptly in comparison to the sweep angle of the wing, in the connection region, so that at least at the leading edge a pronounced sharp bend or kink arises. U.S. Pat. No. 5,348,253 proposes a wing tip extension which uninterruptedly carries on the wing end in a continuous manner by means of a transition region. The transition region is distinguished by a continuous monotonic variation of the cant angle, and there adjoins on to the transition region a straight end region directed from the wing downwardly or upwardly. The sweep angle in the curved transition region thus changes continuously whilst in the end region of the wing tip extension the sweep angle no longer changes, which means that leading edge and trailing edge form straight lines.
The object of the present invention is to provide a wing tip extension for a wing which has a significant reduction of the induced drag and an improvement of the wave drag with only slight increase of the frictional drag in cruise flight without limiting the slow flight characteristics.
This object is achieved by means of a wing tip extension for a wing in accordance with claim 1. The wing tip extension in accordance with the invention has an upper and a lower surface, and a leading and a trailing edge, the geometries of which are of the configuration that between a connection region for connection with a wing and with the tip of the wing tip extension, there are provided a continuous increase in the local dihedral, a continuous increase both of the sweepback angle of the leading edge and also of the trailing edge, and a continuous decrease of the chord of the wing tip extension, and in that the wing tip extension in the connection region carries on from the wing in substance continuously.
The wing tip extension in accordance with the invention thus ensures a reduction of the aerodynamic drag through a significant reduction of the induced drag, and an improvement of the wave drag with only slight increase of the frictional drag in cruise flight, without limiting the slow flight characteristics. The continuous increase of the sweep angle of the leading edge and of the trailing edge and the continuous increase of the local dihedral lead to a non-planar curved geometry. The continuous increase of the local dihedral thereby provides the non-planarity, that is the bending of the wing tip extension out of the plane of the wing upwardly or downwardly, while the continuous increase of the sweep angle, in a view from above the wing tip extension, has the consequence of a curvature to the rear. Through the resulting increased aspect ratio and the non-planarity there is achieved significant reduction of the lift dependent induced drag. The uniform and continuous change of the above-mentioned parameters thereby makes possible an efficient realization of the isobar concept for reduction of the wave drag. Because of the continuous increase of the sweep angle of the leading edge and of the trailing edge, and the increase of the relative nose radius, the slow flight capabilities are not substantially affected, so that the provision of additional leading edge slots on the wing tip extension is not necessary. The significant tapering of the wing tip extension resulting from the increase of the sweep angle of the leading edge and of the trailing edge reduces the additional friction brought about by the additional area of the wing tip extension to an acceptable level.
An important feature of the wing tip extension in accordance with the invention is the configuration of the connection region such that there results a substantially continuous union with the wing concerned. There are thus edges and kinks or sharp changes neither in the connection of the upper and the lower surfaces of the wing tip extension with the corresponding surfaces of the wing, nor between the leading and trailing edges. By means of the continuous transition there is achieved a largely elliptical circulation distribution along the span of the wing, which has as a consequence a significant reduction of the induced drag. Also for transonic characteristics it is of advantage that the span-wise distributed parameters have a continuous and smooth development. Here, the continuousness of the increase of the local dihedral is in particular decisive. A bending of the wing tip extension which is as gentle and continuous as possible, that is a continuous increase of the local dihedral, has a very favorable effect on flight in the transonic region. By means of the configuration of the wing tip extension in accordance with the present invention, in particular through the harmonic continuation of the wing geometry and the simple geometry of the wing tip extension, compressibility effects are reduced to a minimum. The harmonic configuration of the wing tip extension, that is the continuousness of the significant geometrical and span-wise distributed parameters, through the consequently harmonically distributed aerodynamic forces and moments, make possible a significantly weight reduced structural realization. Further, aeroelastic effects are reduced to a minimum by means of the geometrical configuration in accordance with the invention.
Advantageous configurations of the wing tip extension in accordance with the invention are indicated in the subclaims. Advantageously, the overall height of the wing tip extension is 30 to 60% of the span of the wing tip extension. Additionally to the above discussed geometrical parameters it is of advantage if the relative profile thickness of the wing tip extension continuously decreases from the connection region to the tip. This measure has effect particularly for improvement of the transonic characteristics. In this case it is of advantage if the reduction of the relative profile thickness from the connection region towards the tip is more than proportional to the span. In other words, the reduction of the relative profile thickness is advantageously not directly proportional to the span of the wing tip extension, but increases towards the tip. Thereby the relative profile thickness can be reduced from 100% in the connection region to 70%-85% at the tip.
Further, in addition to the above-discussed geometrical parameters, it is of advantage if the relative radius of the nose at the leading edge continuously increases from the connection region towards the tip, that is in comparison with the span a more than proportional increase takes place. Further it is of advantage if the relative radius of the nose at the leading edge continuously increases from 100% in the connection region to 200 to 300% at the tip.
Advantageously, the increase of the dihedral from the connection region to the tip is more than proportional to the span. In other words, starting from the connection region, the dihedral varies first slowly and then ever more strongly towards the tip. Here, it is of advantage when the local dihedral increases from about 0 to 10 in the connection region up to 45 to 60 in the tip.
Also in relation to the sweep angle it is of advantage when the increase of the sweep angle of the leading edge, from the connection region to the tip, is more than proportional to the span. The sweep angle varies first slowly and then ever more strongly towards the tip. Advantageously, the sweep angle of the leading edge increases from between 25 to 45 in the connection region up to 70 to 80 in the tip.
Also in relation to the chord it is of advantage when this reduces, from the connection region to the tip, more than proportionally to the span. Here, the chord may reduce from 100% in the connection region to 10 to 20% in the tip.
The wing tip extension in accordance with the present invention can be formed either as an add-on part for a wing or as part of the wing. That is, the wing tip extension in accordance with the invention can be conceived either as an add-on part for an existing wing or can already be included in the basic concept at the beginning of the development of an aircraft.
Advantageously the span of the wing tip extension is between 8 and 12% of the half-span of the wing. Further, the wing tip extension in accordance with the invention is advantageously designed for a speed range between 0.6 and 0.95 Mach. Further it is of advantage if the wing tip extension in accordance with the invention is designed for wing having a sweep angle between 25 and 45.